A cooling program could increase soaking frequency at the feed bunk as ambient temperature increases. One such example of cooling is as follows:
The fans should work continuously when ambient temperature exceeds 68°F. These recommendations are just an example, and each farm has to devise its own specific cooling program taking into consideration environmental factors, barn design, production level, and overall condition of the cows. Close-up (3 weeks) and early lactation cows are the most sensitive to heat stress and need more stringent cooling strategies.
Forage Inclusion on Diets
Due to the intake depression caused by heat stress, nutritionists in general increase the energy density of the diet by increasing the amount of concentrates and reducing the amount of forages in the ration. This is a sound practice if adequate levels of effective fiber are maintained which are necessary to stimulate rumination and maintain adequate rumen pH. Heat-stressed cows reduce rumination and tend to select finer feed particles. This combination of more concentrated rations together with reduced capacity to buffer rumen pH increases the risk of acidosis during hot weather. These changes can be observed in farms by a reduction in milk fat and higher incidence of lameness.
Research has confirmed the importance of forage inclusion in diets of cows under heat stress. In a University of Georgia experiment (West et al., 1999), heat-stressed cows were fed four experimental diets (forage NDF level = 17%, 19.2%, 23.5%, and 24.7%) having a 40-to-60 forage-to-concentrate ratio. Dietary fiber concentration was achieved by partial substitution of corn silage with bermudagrass hay. Forage NDF increased gradually from 17% (without hay) up to 24.7% (22.8% hay) in the ration. Milk yield was highest, 58.1 and 56.8 pounds per day, for cows fed diets with an intermediate concentration of forage NDF (23.5% and 19.2%, respectively), while milk fat percentages increased linearly with forage NDF inclusion rate. It was necessary to include a minimum of 23.5% forage NDF in the diet to maintain milkfat at 3.5% or greater.
Water is among the critical nutrients during periods of heat stress as losses through evaporation increase at higher ambient temperatures (Table 3). Cows prefer to drink water with a temperature between 63° and 82°F. If the water is not cool enough, it further adds to the heat load. Clean, fresh water is also important during the summer because the high loss of body fluids and electrolytes during sweating and panting can increase the risk of cardiovascular problems. It is very important to have water troughs in the shade and to clean them frequently.
Extended periods of high ambient temperature coupled with high relative humidity compromise the ability of cows in lactation to dissipate excess body heat. Cows with elevated body temperatures exhibit lower intakes and milk yields. Supplemental fan cooling in combination with low pressure feed bunk sprinklers can reduce the effects of heat stress on milk production and feed intake. Several key areas of nutritional management should also be considered for complementing environmental cooling during hot weather.
Fernando Díaz-Royón and Álvaro García, Dairy Science Department, South Dakota State University
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